It has long been known that heart attacks occurring in the morning are typically more serious than those that happen at night. While daily variations in stress hormone levels and blood pressure affect cardiac health, these are only part of the picture. There is also the diurnal variation in immune response involved: neutrophils, the body’s ‘first responders’, cause more inflammatory damage in the morning, causing havoc even as they neutralise pathogens.
“They’re the first sentinel, but they come fully loaded,” said Douglas Mann, MD, professor at Washington University School of Medicine in St Louis. “They’re shooting at everything and dumping a lot of toxic granules on the environment. They are indiscriminate in terms of their ability to destroy, and they take out healthy cells in the process.”
But exactly why they are more damaging at night has been a mystery. Now, researchers have found the reason behind this diurnal difference in destructiveness, and also how to tweak the ‘internal clocks’ of these white blood cells so that they cause less damage during sterile inflammation while still protecting against pathogens. Their findings are reported in the Journal of Exploratory Medicine, and are summarised in JAMA news.
Finding the pattern
The researchers, from Spain and Yale University, discovered that the timing of heart attacks significantly affects their severity due to a ‘neutrophil clock’ controlled by circadian rhythms. Neutrophils are more active during the day (activated by the Bmal1 protein) and less active at night (inhibited by the CXCR4 receptor).
Analysing more than 2000 patients with ST-segment elevation myocardial infarction, the researchers found that those who had an MI in the morning suffered worse cardiac damage than those who had them at night. Mouse experiments confirmed this pattern and showed that genetically disabling the Bmal1 protein reduced daytime neutrophil activity, protecting against severe cardiac injury.
This suggests a treatment strategy of tricking neutrophils into remaining in their nighttime inactive state, allowing doctors to reduce inflammation and lessen heart attack damage during daytime hours without compromising the immune system’s ability to fight infections.
Reducing cardiac damage without compromising the immune system
Mice engineered to have high levels of CXCR4 were given a drug compound, ATI2341, which bound to CXCR4 receptors. When heart attacks were induced, the mice showed reduced tissue damage. To test the neutrophils’ pathogen-fighting ability, they were also infected with Staphylococcus aureus or Candida albicans, but the mice were able to overcome the infection – the treated mice even tolerated the Candida infection better than the controls.
Mann explained why controlling the neutrophils was a better option. “Prior trials have tried to neutralise neutrophils or reduce neutrophil numbers entirely,” Mann noted. “But when you get rid of neutrophils, you’re also handcuffing the immune system. Before, it was considered an inevitability that neutrophils killing off infection also meant damaging a lot of tissue.”
The crucial question is of course whether this research in mice can translate to humans.
Luigi Adamo, MD, PhD, director of cardiac immunology at Johns Hopkins University who was not involved in the study, said that the study, one of the first use immune circadian rhythms to modulate inflammation, “offers new insight into neutrophils and a new way to look at this cardiac damage that might even apply to other types of sterile inflammation.”
Adamo struck a note of caution: the extremely low success rate in animal-to-human translation in cardioimmunology. “Immune cells are not always the same when you go from mice to humans,” he said.
Treatment implementation is a major obstacle
Even if this neutrophil clock alteration could be applied to humans, it would be difficult to administer since heart attacks strike without warning.
“If everyone took one of these drugs in the morning when they woke up, maybe it would make heart attacks less severe, but ‘preventive’ means you’re giving it chronically, and I don’t know what would happen with long-term stimulation of that receptor and other cell types,” Mann said. “Their data support the acute application, but in the long term, that’s a whole different story.”
As systemic treatment, the off-target effects of ATI2341 would need to be explored. He also struggled to envision a potential therapeutic solution.
“Today, when you have a heart attack, in most places with hospitals and well-developed health care systems, the patient gets an angioplasty,” Mann said. “The only time this drug could be given would be at the time of reperfusion, when you’re blowing up the balloon and opening up the clot.” Typically, ideal reperfusion timing is within two hours – but neutrophils probably do their damage within a matter of 30 minutes, Mann explained. “It’s a race against time, and I’m curious if [the researchers] can demonstrate that.”
Source: JAMA
